Navigation devices

A method of creating map data including search acceleration data arranged to increase the speed at which a route can be planned across an electronic map comprising a plurality of navigable segments, each navigable segment representing a segment of a navigable route in the area covered by the map, wherein the method comprises: a) reducing the number of navigable segments to be considered in the creation of the search acceleration data by removing navigable segments to form a core network of navigable segments; b) dividing the electronic map into a set of hierarchical regions such that the or each navigable segment is categorized into at least one region in each level of the hierarchy; c) using a time varying function associated with at least some, and generally each, navigable segment of the core network to determine whether that navigable segment is part of minimum cost route to at least one of the regions and recording this determination in the search acceleration data.